Handle newly created tab separately in 'BufWinEnter' event

The 'TabEnter' event is triggered in the middle of the new buffer establishing,
it is not a good time to restore focus to NERDTree.  When 'BufWinEnter'
event is triggered, the new buffer has been successfully loaded, then we
can restore the focus to NERDTree if necessary.
Make sure a NERDTree window gets opened in the new tab

Remove 's:MirrorIfGloballyActive()', use 's:NERDTreeMirrorOrCreate' instead.